45, 44, 36, 32, 31, 26, 22, 21, ?

17

Between 44 and 36, there is a difference of6,

between 36 and 32 a diff. of 4,

betweem 32 and 31 a diff. of 1.

Between 31 and 26 there is a diff. of 6,

26-22=4,

22-1=21.

So, my guess is that the next no. will be 21-4=17.